1. Field of the Invention
The present invention relates to a polymer film producing method. More particularly, the present invention relates to a polymer film producing method in which polymer film traveling from a tentering device can be free from unwanted deformation harmful to high quality as a product.
2. Description Related to the Prior Art
Polymer film, for example cellulose ester film, is produced by solution casting. A die for casting is used to cast dope or solution of polymer on to a support, to form polymer film. The polymer film is stripped from the support, and dried and wound as a web roll of the product film.
JP-A 62-115035 discloses a tentering device. After the polymer film stripped from the support, the tentering device dries the polymer film being transported for the purpose of regulation of the web width or polymer film width, and stretching. This is to raise a flat characteristic, mechanical strength, and optical characteristic of the polymer film.
Examples of the tentering device include a clip type and a pin type according to a difference in the manner of retaining web edge portions of the polymer film. In the clip type, tenter clip mechanisms retain the web edge portions at points positioned internal from edge lines of the polymer film. A polymer film width of the polymer film is regulated by an adjusted orbit and speed of movement. Also, the tenter clip mechanisms apply tension to the web edge portions in the stretching direction.
Internal portions stretched between the web edge portions will not curl because of the tension. However, large curls occur in the web edge portions of the polymer film positioned outside the tenter clip mechanisms. A term of web edge curls is used for this herein. The portions directly contacted by the tenter clip mechanisms are considerably loose, because of not being dried or shrunken.
After the polymer film is released from the tenter clip mechanisms, rollers guide the polymer film being transported. According to the known techniques, it is likely that wrinkles occur on the polymer film in contact with the rollers, due to a difference in the tension between a middle portion and the web edge portions of the polymer film in the direction of the polymer film width. Furthermore, web edge folds or wrinkles are likely to occur continuously or continually upon contact of the web edge curls with the rollers. The polymer film may be broken into pieces if the web edge folds or wrinkles increase to a considerable extent.
Let a term of a releasing position stand for a position where the polymer film is released from the retention of the tenter clip mechanisms. It is effective to reduce a distance between the releasing position and one of the rollers positioned next downstream from the releasing position, and also to reduce a wrap angle of at least one of the rollers in contact with the polymer film in a section downstream from the releasing position. In view of this, JP-A 9-085680 discloses a method in which the web edge curls are kept free from contact with the rollers by reducing a length of the rollers to a value under the polymer film width in a section downstream from the releasing position. Also, a known method suggests slitting away of the web edge curls by use of a cutter or the like. Also, it is conceivable to change a surface roughness and material of the rollers to facilitate slips of the polymer film. This is effective in keeping the web edges decurled.
In relation to the method of reducing the distance from the releasing position to the rollers, the reduction may be impossible to such an extent as to obtain sufficient effects. This is because devices in the system are not suitable for the distance reduction. Also, the rollers must support the polymer film in the entirety of the polymer film width, and must have a diameter in a limited range.
The method of reducing the wrap angle has a problem in that frequent occurrence of the web edge folds or wrinkles. The degree of freedom in disposition of the rollers is low in consideration of the installing structure or space. The wrap angle cannot be determined suitably for practical reasons. Also, the characteristic of the slip of the polymer film causes a problem in likeliness of scratching the polymer film or in creating a zigzag movement of the polymer film.
Also, a problem arises in the method of reducing the length of the rollers, as the traveling of the polymer film will be very unstable, and because the length reduction cannot be applied to one of the rollers positioned on borderlines between the processes or stations due to the sealing function of the roller. Furthermore, a problem lies in the slitting away for decurling. If removal of the web edge portions from the polymer film is intended before a first contact of the polymer film with the rollers, slitting is extremely difficult due to the considerable corrugated form of the web edge portions. Typically when a space for the tentering device is filled with inert gas, operators cannot enter the space for the system. This makes it very difficult to determine or adjust settings of the system, as external management for the unmanned control is required structurally.
Measures conceived so far to solve the problems result in moderating the drying condition, blocking reduction in the film thickness of the polymer film, and blocking enlargement in the polymer film width. No known solutions are consistent to raising productivity or conceiving new products.